The present invention relates to a method for determining a physical property of a material, particularly of an impact-modified plastic.
Rapid determination of a physical property of a plastic is an important capability for developing new materials, for monitoring manufacturing processes and for trouble-shooting customer service problems. Glass transition temperature, tensile strength, flexural modulus, flexural strength, notched IZOD and heat distortion temperature (HDT) are typically used to characterize performance and applicability of a plastic for a particular use. A number of features pertaining to the molecular structure of a plastic, including molecular weight, composition and chemical bonding affect these physical properties.
For this reason, considerable effort has been applied to develop methods to measure plastic molecular characteristics, particularly of rubber-modified plastics. These plastics are composite materials that comprise a rigid matrix characterized by a relatively high Tg with a dispersed low Tg rubbery phase. Composition of matrix and rubber, amount of dispersed rubber, particle size and particle size distribution of the rubber, strength of rubber-matrix adhesion and mechanical relaxation behavior of the rubber all influence the physical properties of these complex materials.
Mechanical relaxation is considered a key variable for optimizing effectiveness of a rubber as an impact modifier. Mechanical relaxation is strongly affected by crosslink density. Crosslink density is a quantitative measure of the number of crosslinks that exist in a given volume in a thermosetting polymer. Insufficiently crosslinked rubbers do not favorably impart toughness to a sufficient level, while over crosslinked rubbers are ineffective at retarding crack propagation.
Crosslink density in a polymer can be determined by pulsed Nuclear Magnetic Resonance (NMR) spectroscopy. Pulsed NMR techniques are used in instruments for the measurement of the type, property and quantity of lattice bound and free, magnetically active nuclei within a sample. Some of the substances and properties that have been measured by NMR techniques are: polymers and copolymers, oils, fats, crystalline materials and moisture, density and melt indices.
Pulsed NMR uses a burst or pulse of energy that is designed to excite the nuclei of a particular species of a sample being measured. the protons, or the like, of such sample having first been preprocessed in an essentially static magnetic field. In other words a precession is modified by the pulse. After the application of the pulse there occurs a free induction decay (FID) of the magnetization associated with the excited nuclei. That is, the transverse magnetization associated with the excited nuclei relaxes back to its equilibrium value of zero.
Several NMR procedures based on spin-spin relaxation time (T2) have been developed for this purpose. A. Guthausen et al., xe2x80x9cAnalysis of Polymer Materials by Surface NMR via the MOUSE,xe2x80x9d Journal of Magnetic Resonance 130, 1-7 (1998) used T2 to measure curing time and aging characteristics of a polyvinylidene difluoride. Fxc3xcillber et al., xe2x80x9cThe effect of crosslinking in elastomers investigated by NMR analysis of 13C edited transverse 1H NMR relaxation,xe2x80x9d Macromolecular Chemistry 197, 581-583 (1996) correlated xe2x80x2H spinxe2x80x94spin relaxation rate (1/T2) to vulcameter torque.
There is a need for a method to measure crosslink density and other material properties in which little or no sample manipulation is required and that can be used at any stage of a manufacturing process
According to the invention, a method is provided that can be used at any stage of a manufacturing process to determine a physical property such as crosslink density. The method requires little or no sample manipulation. The method is independent of rubber particle size and matrix composition. In the method, a physical property of a material is determined by NMR measurement. The method comprises measuring a T1 (spin lattice relaxation time) for a material, measuring a T2 (spin-spin relaxation time) for the material and calculating a value T1/T2 representative of a physical property of the material.
In another embodiment, the invention is a method to determine a physical property of a material, comprising deriving a model of a relationship between a physical property and a value T1/T2 of the material, detecting a signal T1 and a signal T2 of a sample of the material by NMR and comparing a quotient T1/T2 of the sample to the model to determine a physical property of the material.
In a final embodiment, the invention relates to a system to control a process to synthesize a material. The system comprises a processor, a detector and a comparator. The processor stores a model of a relationship between a physical property and a value T1/T2 of the material. The detector detects a signal T1 and a signal T2 of a sample of the material by NMR. The comparator compares a quotient T1/T2 of the sample to the model to determine a physical property of the material.